"In our quest of a great scientific payoff, we are attempting something never done before at speeds and distances that are truly out of this world," he said.

"It's a bullet trying to hit a second bullet with a third bullet, in the right place at the right time, watching the first two bullets and gathering the scientific data from that impact," he told a news briefing in Washington on Thursday.

Mission scientists said they expected to be able to solve a technical glitch that has caused the spacecraft to return blurry images from one of its instruments.

They will use a mathematical process on the images it captures after they have been transmitted to Earth.

Deep Impact has taken six months to travel some 431 million km (268 million miles) from Earth.

The mission shares a title with the 1998 Hollywood film in which astronauts attempt to stop a comet hitting the Earth.

Deep Impact aims to look under the surface of a comet, at material that has remained untouched since the birth of the planets.